Radioactive plating for radioactive foils

ABSTRACT

A radioactive product, and particularly a radioactive foil, comprising a radioactive core and a protective plating disposed over the core comprising gold alloyed with from 0.10 to 25.0 percent by weight of cobalt, cadmium, nickel, copper or silver.

United Statea tent Clarence W. Wallhausen Bernardsville, N.J.;

Walter F. Buck, Bloomsburg, Pa. 654,328

July 19, 1967 Mar. 2, 1971 Nuclear Radiation Developments, lnc. Grand Island, NX.

lnventors Appl. No. Filed Patented Assignee RADIOACTIVE PLATING FOR RADIOACTIVE FOILS 7 Claims, 2 Drawing Figs.

U.S. Cl 250/106, 176/67 Int. Cl G2lh 5/00 Field of Search 250/106 Primary Examiner-Archie R. Borchelt Att0rneyPennie, Edmonds, Morton, Taylor and Adams ABSTRACT: A radioactive product, and particularly a radioactive foil, comprising a radioactive core and a protective plating disposed over the core comprising gold alloyed with from 0.10 to 25.0 percent by weight of cobalt, cadmium, nickel, copper or silver.

PATENTEUHAR zlsn 3567343 FIG. 2

INNENTORS CLA CE W. WALLHAUSEN WAL R F. BUCK BY m"n 4.. a4...

ATTORNEYS RADIOACTIVE PLATING FOR RADIOACTIVE FOILS BACKGROUND OF THE INVENTION The field to which this invention pertains is that of radioactive foils and similar products comprising a radioactive core on which is disposed a metallic sealing layer as a safeguard against contamination. The improvement of the invention resides in a protective plating disposed over the sealing layer comprising an alloy of gold and from 0.10 to 25 .0 per cent by weight of cobalt, cadmium, nickel, copper or silver.

The state of the prior art is illustrated by U.S. Pat No. 2,476,644 to Wallhausen et al. which describes a radioactive metal foil comprising a radioactive core on which is disposed a metallic sealing layer and an outer protective plating which imparts a wear-resistant and noncorrosive surface to the foil. This second or outer coating is specified in the aforementioned patent to be of nickel, cobalt, chromium, cadmium, copper, tin, silver, gold, rhodium, platinum, iridium, brass or cupronickel, with nickel or chromium preferred. These measures were considered sufficient many years ago to prevent emission of radioactive gas but that problem. no longer confronts the art. All of the early techniques of this sort are quite useless in meeting current safety standards against contamination due to oxidation, flaking, edge cutting and so on.

In recent times, the greater part of the vast quantities of radioactive foil of this general type has had an exterior plating of palladium. It has proved superior to the various other platings mentioned above, but nonetheless has been far from totally satisfactory on the critical matter of contamination.

Platings of palladium not only tend to flake from the foil but they are difficult to deposit in uniformly thick layers. Also, chipping and cracking of palladium platings is not uncommon when the foil is out.

SUMMARY OF THE INVENTION It has been discovered that if gold is alloyed with from 0.10 to 25.0 per cent of cobalt, cadmium, nickel, copper or silver a plating metal far superior to palladium results as an exterior coating for radioactive foils. Such alloys are more abrasion resistant, they adhere better to the underlying layers, they are less susceptible to scratching or acid erosion, and they produce a better sealing effect over the exposed edges when cut. On the other hand, it has also been discovered that the alloys of gold contemplated for the exterior plating in accordance with this invention are far superior to commercially pure gold suggested years ago for gas-impervious layers, because the alloys are considerably harder and afford a protective shield much less vulnerable to abrasion and scratching. Public health authorities look unfavorably on pure gold platings because they are so soft that they readily break through during wipe tests.

BRIEF DESCRIPTION OF THE DRAWING The accompanying single sheet of the drawings illustrates in cross section, on a greatly magnified scale, the tiered structure of a radioactive foil containing the improved protective plating of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The initial steps in fabricating the radioactive product of the invention in the form of a foil are known in this art. The radioactive core is formed, for example, by mixing or coprecipitating a radioactive salt and a gold powder. This particulate radioactive mixture is then compressed into a solid block by heat and pressure and rolled into a radioactive sheet. Gold foil may be laminated to each side of the radioactive sheet by a rolling process and a backing foil of silver or gold is usually laminated in the same manner to contribute strength. Together these components form a foil which may have an overall thickness of about .005 inch with the silver backing being the thickest of the layers. The result is a foil strip of laminated construction having an interior radioactive layer covered on each side with a sealing layer of gold with a relatively heavy surfacing of silver on one side of the foil to serve as a reinforcing backing element. Such foils find widespread industrial use in static eliminators, as discharge-stabilizing elements in electron tubes and as radiation sources in beta and gamma gauges, in photometry, emergency marking devices, and so on.

It has been known, however, that foils of this sort are more satisfactory if a plating of a protective metal is electrodeposited over its exposed surface, particularly to render the otherwise exposed gold and silver of the foil resistant to abrasion, corrosion, oxidation and other forms of chemical or physical attack which results in removal of radioactive material. Palladium has been the most common plating metal for guarding against contamination of this: sort.

The present invention is predicated on the discovery that a substantial improvement over palladium plating can be achieved by employing as the plating metal an alloy of gold and from 0.10 to 250 per cent by weight of cobalt, cadmium, nickel, copper or silver. Preferably, the plating alloy should consist ofgold and from 0.1 to 4.0 per cent by weight of cobalt or cadmium. These preferred plating alloys exhibit a hardness range comparable to that'of palladium. Cobalt and cadmium in the small amounts noted above are preferable. in the gold plating alloy because they do not chip, crack, or peel and they are exceptionally resistant to corrosion.

As shown in FIG. 1 of the drawing radioactive core layer 10 is enveloped on both surfaces by plating layers 12 and 14 of the gold alloy of the present invention and backing foil 16 is laminated to .the surface of protective plating 14. In FIG. 2 there is shown gold foil sealing films 1:8 and 20 forming a first sealing layer about radioactive core 10.

Comparative tests have been'conducted to determine the relative performance of foils differing only in that some have the conventional palladium over-plating while the others have an alloy of gold and 0.1 to 4.0 per cent cobalt or cadmium for that purpose. Under equal attack from nitric acid, the gold alloy plated foil shows no adverse effects after a treatment which causes the palladium plating to evidence breakage, lifting from the underlying layers and dissolution of those layers, In an adhesion test, the palladium plating evidences separation from its underlying layers while the gold alloy plated foil remains unaffected. The palladium plated foil in a scratch test evidences brittleness and a tendency toward chipping and cracking whereas the gold alloy plated foil is undamaged. In an abrasion test, the palladium plating becomes visibly scratched and in some places broken through whereas the gold alloy plating is simply worn down smoothly. There is firm evidence also that when the respective foils are cut the gold alloy plating tends to seal off the exposed cut edge more satisfactorily than the conventional palladium plating and hence permits less radioactive contamination. The reason for this is the markedly superior malleability of the gold alloy as compared to palladium which permits it to deform over the cut edge and decrease the exposed area of radioactive material. Under comparative tests subjecting the foils to ammonium sulfide, hydrogen sulfide, sulfurous acid and nitric acid, the palladium plated foil evidences a loosening of its layers while the gold alloy plated foil resists the effects quite well.

It is by reason ofthe fact that the gold alloy plating bonds so firmly to the underlying layer, in the manner of a solid solution, that these advantages are achieved. The practical result is a radioactive product which creates the very least danger of contamination even under the most stringent modern standards of safety imposed in this field.

Variations in the structure of the radioactive product of the invention will be quite apparent. For example, the gold alloy plating maybe applied to either or both sides of the foil whether or not a silver backing layer is employed. Also, for certain special uses, platinum may be used instead of gold or silver in the inner layers of the foil, but not of course for the improved protecting plating.

We claim:

1. In a radioactive core, an improved protective plating disposed over said core comprising an alloy of gold and from about 0.10 to 25.0 per cent by weight of an element selected from the group consisting of cobalt, cadmium, nickel, copper and silver.

2. A radioactive product according to claim 1 wherein the plating alloy consists essentially of gold and from 0.1 to 4.0 per cent by weight of cobalt or cadmium.

3. A radioactive product according to claim 1 wherein a metallic sealing layer is disposed over the core and the protective plating is disposed over the sealing layer.

4. A radioactive product according to claim 3 in the form of a foil wherein the core is a sheet and the sealing layerris laminated to and covers the opposite sides of the core sheet,

said plating covering all exterior surfaces of the foil.

5. A radioactive product according to claim 4 which also includes a silver or gold backing layer laminated to and reinforcing the remainder of foil. I

6. A radioactive product according to claim 4 wherein the metallic sealing layer is of gold.

7. In a radioactive foil comprising a radioactive core sheet on the opposite sides of which are laminated at least one gold sealing layer and a silver or gold backing layer, an improved protective plating covering the exterior of the foil comprising an alloy of gold and from 0.1 to 4.0 percent by weight of cobalt or cadmium. 

1. In a radioactive core, an improved protective plating disposed over said core comprising an alloy of gold and from about 0.10 to 25.0 per cent by weight of an element selected from the group consisting of cobalt, cadmium, nickel, copper and silver.
 2. A radioactive product according to claim 1 wherein the plating alloy consists essentially of gold and from 0.1 to 4.0 per cent by weight of cobalt or cadmium.
 3. A radioactive product according to claim 1 wherein a metallic sealing layer is disposed over the core and the protective plating is disposed over the sealing layer.
 4. A radioactive product according to claim 3 in the form of a foil wherein the core is a sheet and the sealing layer is laminated to and covers the opposite sides of the core sheet, said plating covering all exterior surfaces of the foil.
 5. A radioactive product according to claim 4 which also includes a silver or gold backing layer laminated to and reinforcing the remainder of foil.
 6. A radioactive product according to claim 4 wherein the metallic sealing layer is of gold.
 7. In a radioactive foil comprising a radioactive core sheet on the opposite sides of which are laminated at least one gold sealing layer and a silver or gold backing layer, an improved protective plating covering the exterior of the foil comprising an alloy of gold and from 0.1 to 4.0 percent by weight of cobalt or cadmium. 